Reporting from San Francisco — A federal trial on same-sex marriage focused Wednesday on the similarities and differences between homosexual and heterosexual couples, with a psychology professor citing "remarkable similarities."

Letitia Peplau, an expert on couple relationships, testified that studies have found that the quality of heterosexual and homosexual relationships was on average "the same" as measured by closeness, love and stability.

"On average, same-sex couples and heterosexual couples are indistinguishable," said Peplau, a UCLA professor of social psychology called by attorneys for two same-sex couples who are trying to overturn Proposition 8, the 2008 voter initiative that reinstated a state ban on same-sex marriage.

Peplau cited a survey of Californians in which 61% of lesbian respondents said they were living with a partner compared with 46% of gay men and 62% of heterosexuals.

Homosexual couples tend to have shorter relationships than married couples, she said, but so do unmarried cohabiting heterosexuals.

Under cross-examination, Peplau acknowledged that gay men value monogamy less than lesbians and heterosexuals of both genders.

Among heterosexuals and lesbians, "monogamy correlated for relationship satisfaction," Peplau said during the trial's third day. But "for gay men there is no association between sexual exclusivity and the satisfaction of their relationship."

She said several studies have found that married people tend to have better health than non-married people. She cited a government report that said married individuals are less likely to smoke or drink in excess and report fewer health problems than singles, and attributed the disparity in part to the support spouses share.

She also told of a recent Kaiser Family Foundation report in which 74% of homosexuals said they would like to marry some day.

During cross-examination, Nicole J. Moss, an attorney for the Proposition 8 campaign, put into evidence government statistics from Belgium and the Netherlands, where same-sex marriage is legal and, Moss said, a substantially smaller percentage of gays and lesbians chose to marry than heterosexuals.

Asked about those figures, Peplau said she was "struck" that so few same-sex couples would marry in Europe while so many want to marry in the U.S. A lawyer for the plaintiffs later suggested that the smaller proportion of married homosexuals in those countries might stem from the fact that marriage is a relatively new opportunity for them.

Moss also asked Peplau if married people might have better health because they have greater access to healthcare, which domestic partnerships also provide by requiring employers to insure partners.

Peplau said there was "no question" that domestic partnerships have improved the lives of same-sex couples, but said those partnerships are "not equivalent" and do not convey all the merits of marriage.

Moss also tried to challenge Peplau's contention that heterosexual and homosexual couples are extremely similar.

She suggested that one benefit of marriage is that couples do not have children out of wedlock, and asked Peplau if she agreed that gay men and lesbians do not have children accidentally.

Peplau sputtered, then said: "Can two lesbians spontaneously impregnate each other? Not to my knowledge, no." The courtroom filled with laughter.

Earlier in the day, a Proposition 8 attorney got Yale historian George Chauncey to say that gays and lesbians have become politically and socially more powerful in recent years. But Chauncey also said that discrimination persists and described writings by a Proposition 8 proponent as evidence of long-held and inaccurate negative stereotypes.